In the use of nuclear fuel, it is important to know the measurement of the length of a fuel column, that is the end-to-end length of the fuel inside each nuclear fuel rod, both before and after service of the fuel rod in a nuclear reactor. Such knowledge will enable verification that the fuel column length in the assembled fuel rod complies with established product specifications and also provides insight into the expected as well as actual performance of the fuel during service in the nuclear reactor. By knowing the before and after service length of the fuel column in the rod, the foregoing information on performance will be ascertainable.
The final fuel column length is equivalent to the overall length of the fuel rod which may be measured manually by means of a micrometer gauge, minus the sum total of the plenum chamber length and the upper and lower end cap lengths. The end cap lengths are known and can readily be obtained from the fabrication specifications, thus leaving only the plenum chamber length as an unknown.
Present techniques used to determine the plenum chamber length of the fuel rod during a pre-service inspection of the fuel rod include taking an X-ray of the upper portion of the fuel rod. The use of the X-ray measurement technique is not readily adaptable for use in the inspection basin required for post-service rod measurements and, accordingly, there is a need for a fast, accurate and reliable measurement technique for determining the plenum chamber length both for pre-service and post-service fuel rods.